Stripline couplers consist generally of a pair of adjacent transmission line conductors located within one or more substrates positioned between one or more ground planes. The transmission line conductors may be coplanar or non-coplanar.
A directional coupler couples a certain amount of power input to a first transmission line to a second transmission line. The ratio of the power input to the first transmission line to the power coupled to the second transmission line is referred to as the coupling factor. For example, a directional coupler having a 10 db coupling factor couples one-tenth of the input power to the coupled port of the second transmission line (and theoretically transmits the other nine-tenths of the input power to the output of the first transmission line). Directional couplers are useful as a power dividing circuit and as a measurement tool for sampling RF and microwave energy
The directivity of a directional coupler refers to the ratio of the power measured at the forward-wave sampling terminals, with only a forward wave present in the transmission line, to the power measured at the same terminals when the direction of the forward wave in the line is reversed. Directivity is usually expressed in decibels (dB). High directivity in directional couplers is usually attained by manufacturing the transmission line to have a predetermined characteristic impedance (determined by the dimensions of the strip conductor, dielectric constant of the substrate and thickness of the substrate) that matches the source impedance and/or load impedance. As such, any variations in the value of the characteristic impedance of the transmission line with respect to a source and/or load impedance degrades directivity.
Typically, in order to achieve high directivity (i.e. manufacturing the transmission line with a precise characteristic impedance--usually fifty ohms), directional couplers are manufactured using expensive substrate material (dielectric medium). Such microwave laminates, as they are commonly referred to, require special manufacturing techniques to inlay the laminate on a conventional printed circuit board. Additionally, the dielectric constant (Er) and thickness of the substrate are tightly controlled which produces a transmission line having a relatively precise characteristic impedance, thus enhancing the directivity of the directional coupler. Tight control of substrate parameters (dielectric constant, thickness, etc.) increases the cost of the directional couplers.
Accordingly, there exists a need for a directional coupler having high directivity and capable of manufacture on conventional printed circuit boards using substrates that are commonly used with conventional circuit boards. Further, there is a need for a directional coupler that allows use of less expensive substrate material that can be manufactured with higher tolerances, thus allowing the directional coupler to be manufactured on basic printed circuit boards.